Animal

An animal is any creature which is a member of the Animalia kingdom. Animals must eat plants or other organisms to survive. The plants are called "producers" and the animals are "consumers". Scientists call consumers heterotrophs. Animals have complex eukaryotic cells, unlike bacteria which have much simpler prokaryotic cells. Animal cells do not have cell walls, only structural proteins in the cell membrane; this allows the animal cell a certain amount of flexibility.

The simplest of all animals are sponges, which do not move. Sponges have different cells for different tasks, but these cells are not organized further into tissues or organs. Sponges also do not exhibit symmetry, unlike all other animals.

The next simplest animals comprise a single species, which fall somewhere between sponges and all other animals. This species, Trichoplax adhaerans, is very small, yet has cells organized into different kinds of tissues, though it does not have organs. This is an improvement on the sponge body plan.

True Animals are grouped into a sub-kingdom called Eumetazoa. All True Animals have tissues and organs, except for one type called Myxozoa. These have regressed to a single-celled form.

The simplest eumetazons are the Radiata, a loose group that includes jellyfish, corals, and comb jellies. Most of them belong in the phylum Cnidaria, except for the comb jellies, which comprise the phylum Ctenophora. Radiatans have what is called radial symmetry, where they can be divided into two even halves in many planes, like a wheel or a circle. All cnidarians have special cells called cnidocytes, which produce the sting. Comb jellies have no such cells, and are harmless to humans.